Vacuum furnace

ABSTRACT

In the vacuum furnace disclosed herein, heat losses through radiation are reduced by a radiation shield of novel construction. The shield is formed of a spiral of a foil of metal in which the adjacent turns of the foil are separated by a sparse scattering of a relatively nonconducting material such as a ceramic powder.

United States Patent William P. Teagan Inventor Billerica, Mass. Appl. No. 844,942 Filed July 25, 1969 Patented Mar. 16, 1971 Assignee Thermo Electron Corporation Waltham, Mass.

VACUUM FURNACE 11 Claims, 5 Drawing Figs.

(1.8. CI 13/31, 13/35, 263/50 Int. Cl 05b 3/00, F27d 1/00 Field of Search 263/40 (Cursory), 50; 13/31, 35, 26 (Cursory); 106/57 (Cursory), 65 (Cursory) [56] References Cited UNITED STATES PATENTS 2,308,945 l/1943 Van Embden.; 13/26 3,257,492 6/1966 Westeren 13/31 3,409,730 11/1968 Ebihara 13/35 Primary ExaminerBernard A. Gilheany Assistant Examiner-Roy N. Envall, Jr. Attorney-Kenway, Jenney & Hildreth ABSTRACT: In the vacuum furnace disclosed herein, heat losses through radiation are reduced by a radiation shield of novel construction. The shield is fonned of a spiral of a foil of metal in which the adjacent turns of the foil are separated by a sparse scattering of a relatively nonconducting material such as a ceramic powder.

Patented March 16, 1971 3 Sheets-Sheet 1 FIG. I

INVENTOR WILLIAM P. TEAGAN 21 MVL/MM ATTORNEYS Patented March 16, 1971 I 5 Sheets-Sheet 2 ATTORNEYS Patented March 16, 1971 5 Sheets-Sheet 3 008 oood oom OONJ SilVM 'iflclNl UBMOd INVENTOR WILLIAM F? TEAGAN ATTORNEYS vAcUUM FURNACE BACKGROUND OF THE INVENTION This invention relates to high-temperature furnaces and more particularly to a vacuum furnace which is radiation shielded.

In various high-temperature furnace constructions proposed heretofore, radiation shielding, if provided at all, has typically been'in the form of a plurality of discrete shields which are rigid and self-supporting. The number of such shields has typically been limited by the bulk and size they add to the furnace in view of the thicknesses of the shields themselves and the spacing required for rigid support. Further, such prior art shielding arrangements have required a substantial amount of material in the shields themselves and the cost of this material may substantially raise the price of furnaces designed for high-temperature operation.

Among the several objects of the present invention may be noted the provision of a high-temperature furnace employing radiation shielding; the provision. of such a furnace in which the shielding is relatively compact; the provision of such a furnace in which heat losses are relatively small; the provision of such, a furnace which requires a relatively low power input to achieve a given temperature; the provision of such a furnace which is relatively inexpensive; and the provision of such a furnace which is easily constructed.

Other objects and features will be in' part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION Briefly, a radiation-shielded furnace according to the present invention involves chamber defining means including a multitum spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting material separating adjacent turns of the foil. The first turn of the spiral encompasses a volume which forms the heating chamber of the furnace. Each open end of the spiral is closed by a respective cover and a heater is provided for generating heat in the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevation in section of a vacuum furnace according to this invention;

FIG. 2 is a sectional view of the sidewall of the furnace taken substantially on the line 22 of FIG. ll;

FIG. 3 is a view, taken substantially on the line 3-3 of FIG. ll showing the upper cover with heater;

FIG. 4 is a sectional view illustrating another embodiment of the invention; and

FIG. 5, is a plot comparing the temperature/power characteristics of a furnace according to this invention with a conventional Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, the furnace illustrated there is of generally cylindrical configuration and comprises a generally tubular member it closed at each end by a respective cover t3, and 15. As may be seen in FIG. 2, the sidewall-defining tubular member It includes an outer metallic shell 17 and a radiation shield 19.

As may be seen in FIG. 2, the radiation shield 19 comprises a spiral 21 of a foil of a suitable metal with adjacent turns of the foil being separated or spaced by a sparse scattering or dusting of discrete particles 23. Particles 23 are preferably of a relatively nonconducting material such as a ceramic powder. As will be readily understood, such a radiation shield may be constructed by scattering a ceramic powder on the surface of a strip of foil as it is wound up on a suitable form.

Construction in this manner permits the use of a multiplicity of radiation shield elements, i.e. the individual turns of the foil, without adding a large bulk to the furnace. The particles space and support the individual layers of foil locally and thus permit a thin foil to be used rather than discrete, relatively heavy sheets or plates. Thus, for the purpose of the present invention, the term foil as used herein and in the claims should be understood to mean a layer of metal which is not more than 0.005 inches thick. For high-temperature vacuum furnace applications, a refractory metal such as tungsten, molybdenum or tantalum may be employed for the foil 21 while furnaces for use at lower temperatures, e.g. annealing furnaces, may be constructed with a foil of a metal such as titanium, stainless steel or nickel.

The covers 13 and 15 are constructed in an analogous manner and comprise a backing plate 31 and a radiation shield 33. The shields 33 may also comprise multiple layers of foil, the layers being spaced by a ceramic powder and being wired to the backing plate 31.

Heat is generated within the chamber of the furnace by means of a resistance heater 35 which comprises a pair of strips of tungsten wire screen 37 and 39. Each of the strips is draped between a respective input electrode 41 and 43 and an intermediate support electrode 45, the electrodes being mounted on respective feedthrough terminals 47-49. This heater arrangement has the advantage that the heating chamber is left relatively clear for the insertion of a workpiece. A relatively simple heater of this type may be employed since losses from the furnace are relatively'low as compared with conventional constructions. In other words, a relatively high temperature can be obtained with a relatively low power input. In addition, longlife is obtained, heaters of this type having been operated for hundred of hours at 2000 C. Tungsten wire screen suitable for use as the resistance heating elements 37 and 39 is readily available for other purposes and thus special purpose heating elements need not be custom fabricated at considerable expense. Alternatively, wire mesh heating elements can be hung alongside the periphery of the tubular member Ill with suitable electrodes being provided at v the ends of the strips for interconnection and for the application of electric power.

In operation, the furnace is mountedon legs 5! within a conventional bell jar 53 which is then evacuated by means of a pump as indicated diagrammatically at 55. In one example ofa furnace constructed substantially as illustrated, the radiation shield I9 comprised 20 layers or turns of molybdenum foil and 20 layers of tungsten foil. The foils were 0.001 inches thick and adjacent layers were separated by sparsely distributed zirconia oxide powder of size such that the resulting average thickness for each radiation shielding layer was about 0.0025 inches. Thus, the total thickness of the radiation shield 19 was only about 0.1 inches thick. The heat losses from this furnace were low enough so that a temperature of 2,000 C. was

achieved with a power input of less than 2,000 watts, even,

though the heating chamber was approximately 5 inches in diameter by 6 inches high. A plot of temperature as a function of input power for this furnace is indicated at A in FIG. 5. For comparison, there is indicated at B the temperature/power characteristic of a conventional furnace having a heating chamber of comparable size, i.e. 4- inches in diameter by 7 inches high. This conventional furnace is radiation shielded by means of five self-supporting tungsten shields which together form a shield wall about l /sinches thick. It can thus be seen that relatively efficient operation was provided by the present invention in a relatively compact structure. Further, the application of power for heating was facilitated by the use of the wire mesh heater described previously which presented a relatively high impedance to the power supply, e.g. about 1 ohm at L800 C. The heating elements were constructed of standard,

tungsten screen with a wire size of 0.004 inches and a 35 wire per inch mesh.

While a furnace of circular cross section has been shown by way of example, it should be understood that other cross-sectional shapes could be used. Therefore, as used herein and in the claims, the term spiral should be understood to include wound radiation shielding foil when the overall cross section is other than circular, e.g. rectangular.

An alternative radiation shield construction is illustrated in the sectional view of FIG. 4. This sectional view is taken along a plane parallel to the axis of the respective spiral rather than transversely thereto as with FIG. 2. In FIG. 4, the adjacent turns of a spiral of a metal foil 61 are separated by fine wires 63 of relatively nonconducting material, either metallic or nonmetallic which are wound up in spirals with the foil 61. The separation between the adjacent fine wires 63 in each layer is preferable somewhat random so that the wires is adjacent layers do not lie in alignment with one another. Although each turn of the wires 63 can conduct heat locally from one layer to the next and thereby effectively short circuit that one radiation shield layer, it will be seen that the total effect on the overall radiation shield is quite small since the heat thusly transmitted by one wire at a given point does not as readily penetrate any of the other layers. It will be understood 1 that the lateral paths along the foil are relatively long as com- 'pared with the transverse dimensions and are thus inefficient conduction paths. As with the first example, the randomly spaced fine wires locally space and support the adjacent turns of the foil 61 so that a relatively thin foil, which would not be self-supporting, may be used.

In view of the foregoing, it may be seen that several objects of the present invention are achieved and other advantageous results have been attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it should be understood that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

lclaim:

l. A radiation-shielded furnace, said furnace comprising:

chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber;

a cover for each open end of said spiral for closing said chamber; and

a heater for generating heat in said chamber.

2. A radiation shielded furnace, said fumace comprising:

chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber;

a cover for each open end of said spiral for closing said chamber; and

a heater including at least a pair of electrodes and a woven wire mesh resistance heating element extending between said electrodes for generating radiant energy in said chamber.

3. A radiation'shielded vacuum furnace, said furnace comprising:

chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting ceramic material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber;

a cover for each open end of said spiral'for closing said chamber;

a heater including at least a pair of electrodes and a woven wire mesh resistance heating element extending between said electrodes for generating radiant energy in said chamber; and

means for evacuating a space including said furnace.

4. A furnace as set forth in claim 3 wherein each of said covers includes a plurality'of layers of a refractory metal foil separated by a sparse scattering of discrete particles of a ceramic powder.

5. A furnace as set forth in claim 3 wherein said foil is less than 0.005 inches thick.

6. A furnace as set forth in claim 3 wherein said foil is molybdenum.

7. A furnace as set forth in claim 6 wherein said ceramic material is zirconia oxide.

8. A furnace as set forth in claim 7 wherein said spiral includes in the order of 40 turns.

9. A furnace as set forth in claim 3 wherein said mesh is tungsten.

10. A furnace as set forth in claim 1 wherein said spacer ele' ments are fine particles of a ceramic powder.

ll. A furnace as set forth in claim 1 wherein said spacer elements are a single layer of widely spaced turns of fine wire wound up with said foil, the wires in adjacent layers being offset from one another. 

1. A radiation-shielded furnace, said furnace comprising: chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber; a cover for each open end of said spiral for closing said chamber; and a heater for generating heat in said chamber.
 2. A radiation shielded furnace, said furnace comprising: chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber; a cover for each open end of said spiral for closing said chamber; and a heater including at least a pair of electrodes and a woven wire mesh resistance heating element extending between said electrodes for generating radiant energy in said chamber.
 3. A radiation-shielded vacuUm furnace, said furnace comprising: chamber-defining means including a multiturn spiral of a foil of a refractory metal with a sparse scattering of discrete particles of a relatively nonconducting ceramic material separating adjacent turns of said foil, the first turn of said spiral encompassing a volume which forms said chamber; a cover for each open end of said spiral for closing said chamber; a heater including at least a pair of electrodes and a woven wire mesh resistance heating element extending between said electrodes for generating radiant energy in said chamber; and means for evacuating a space including said furnace.
 4. A furnace as set forth in claim 3 wherein each of said covers includes a plurality of layers of a refractory metal foil separated by a sparse scattering of discrete particles of a ceramic powder.
 5. A furnace as set forth in claim 3 wherein said foil is less than 0.005 inches thick.
 6. A furnace as set forth in claim 3 wherein said foil is molybdenum.
 7. A furnace as set forth in claim 6 wherein said ceramic material is zirconia oxide.
 8. A furnace as set forth in claim 7 wherein said spiral includes in the order of 40 turns.
 9. A furnace as set forth in claim 3 wherein said mesh is tungsten.
 10. A furnace as set forth in claim 1 wherein said spacer elements are fine particles of a ceramic powder.
 11. A furnace as set forth in claim 1 wherein said spacer elements are a single layer of widely spaced turns of fine wire wound up with said foil, the wires in adjacent layers being offset from one another. 